Ground fault circuit interrupters (GFCI) for interrupting the flow of electrical power to a device upon the occurrence of a ground fault have been known for many years. Known devices are usually effective in detecting ground faults of the type caused by current accidentally flowing through a human body. Such currents are normally in the low range of 6 mA to about 264 mA. However, other types of ground faults, such as the types caused by miswiring or inrush currents may produce high currents on the order of 4 amps to 120 amps or more that might not be reliably detected by known GFCI's .
Conventional ground fault circuit interrupters usually employ a transformer having a toroidal core of magnetic material through which the hot and neutral wires of the electrical circuit flow to form a differential primary and about which is wound a secondary winding, often about 1,000 turns, from which the ground fault signal is detected. Typically, the secondary winding is connected to the input of a sensitive operational amplifier, which is arranged with suitable feedback and output components to generate a trigger signal in response to a ground fault for opening contacts in the primary circuit for interrupting the flow of power.
The differential toroidal transformer arrangement used in conventional ground fault circuit interrupters is effective over the low range of ground faults commonly experienced, but may be less effective or ineffective in detecting very high current ground faults.
Ground fault currents on the low end of the human physiology range, such as approximately 6 mA ground faults result in magnetic flux in the toroidal core well below the saturation range. Currents on the high end of the human physiology range, in the approximately 264 mA range may produce some saturation of the transformer, but generally result in an output signal at the secondary, whose waveshape is a reasonable facsimile of the input signal. Non-human physiology based ground faults that produce currents of 4 amps to 120 amps or more, completely saturate the transformer, and produce high voltage extremely short duration pulses on the secondary winding. Integrating ground fault detector circuits attached to the secondary winding adequately detect ground fault currents in the human physiology range, both the upper and lower ends of the range, but may be insensitive to the extremely short duration high voltage pulses produced when the transformer is driven into saturation by high current ground faults.
It is an object of this invention to provide an improved ground fault circuit interrupter that substantially maintains the sensitivity to ground faults in the human physiology range, while greatly increasing sensitivity to high current ground faults outside this range.
It is another object of this invention to provide an improved ground fault circuit interrupter with enhanced operating range that is not significantly more complex than existing ground fault circuit interrupters.
It is still another object of this invention to provide a ground fault circuit interrupter with enhanced operating range that is not significantly more expensive than existing ground fault circuit interrupters.
Briefly stated, and in accordance with a presently preferred embodiment of the invention, a ground fault circuit interrupter comprises a transformer having a saturable core; a secondary winding on the core; an integrating detector coupled to the secondary winding; and a pulse stretcher connected between the secondary winding and the differential detector for stretching high voltage short duration pulses sufficiently to allow them to be detected by the detector.